Various types of spinal column disorders are known and include scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal backward curvature of the spine), excess lordosis (abnormal forward curvature of the spine), spondylolisthesis (forward displacement of a lumbar vertebrae), and other disorders such as ruptured or slipped discs, broken or fractured vertebrae and the like. Patients who suffer from such conditions usually experience extreme and debilitating pain. A technique known as spinal fixation uses surgical implants which mechanically immobilize areas of the spine during surgical fusion of the treated vertebrae. Such techniques have been used effectively to treat such conditions and in most cases to bring the patient relief from pain.
One particular technique for spinal fixation includes the immobilization of the spine by the use of a pair of spine rods that run generally parallel to the spine. Bone screws are used as the anchor points for the spine rods and clamps are used to fasten the spine rods to the bone screws. Bone screws are generally placed two per vertebrae, one at each pedicle on either side of the spinous process with a protruding threaded end for anchoring the connector. Various different connectors have been developed for fastening a spine rod to a bone screw. Saddle clamps comprising a pair of mating half saddles that fit over the protruding threaded end of a bone screw and that are clamped together by a nut threaded to the bone screw are commonly used. Examples of such devices can be found in U.S. Pat. Nos. 4,653,481 and 5,030,220. These patents further disclose the use of spine rods and clamps that include longitudinal serrations which improve the grip between the spine rod and the clamps.
Other devices include a one-piece connector that has a first opening for receiving a spine rod and a second oblong opening for fastening the connector to a bone fastener with a threaded nut. A set screw can be threaded into a threaded aperture in communication with the first opening, to lock the rod in place. Such a connector is disclosed in U.S. Pat. No. 5,129,900.
Another clamp includes an articulating joint and is disclosed in U.S. Pat. No. 5,053,034. A first block is fastened to a bone screw and a second block with a protruding cylindrical arm fits into an aperture in the first block to form the articulating joint. The angle between the blocks can be adjusted by rotating the second block with respect to the first block at this joint. Once the desired angle is obtained, a set screw is threaded into a second aperture, in communication with the first aperture in the first block, to lock the two blocks together. A pair of such articulating joints are fastened to a pair of vertebrae and the two are fastened to each other by a "screw spindle" or other rigid connecting member.
There are several inadequacies with these devices. For example, the clamps often do not provide sufficient grip to prevent the spine rod from slipping with respect to the clamp. For some clamps, the clamp tends to slip with respect to the bone screw. This problem is especially apparent with laterally adjustable clamps. Furthermore, most of these devices are fairly bulky and portions of the clamp extend to high above the patient's spine which can lead to irritation of the patient's muscles and other tissue that rub against the clamp when it is installed. Moreover, some clamps tend to include several parts that must be assembled during surgery. Complicated assembly onto the spine increases the possibility that parts will be dropped or misplaced during surgery.
An improved one-piece clamp is desired that provides a strong and secure connection between a bone screw and a spine rod. Such a device should be easy to install and should include relatively few parts. It is also desirable that a one-piece clamp have a low profile so as to minimize irritation of the patient's surrounding muscles and other tissue.